1. Field of the Invention
The present invention relates to a thin film transistor liquid crystal device (TFT-LCD) and, more particularly, to a source driver of a liquid crystal display (LCD) having a multi-scan function and a method for driving the same.
2. Discussion of the Related Art
In multi-scanning, video signals of low resolution (low video mode) can be enlarged in a vertical direction on an LCD panel of high resolution. Or, in the case of a LCD panel of low resolution, a video source may have its resolution reduced so that it may be displayed on the LCD panel. In the latter case, some of the video source data may be removed.
Enlargement of video signals in a horizontal direction can be easily achieved by increasing a sampling rate. In contrast, enlargement of video signals in the vertical direction, achieved in a method in which picture data are stored utilizing frame memories, isn't easily achieved.
In a conventional source driver of an LCD, image signals of resolution suitable for a corresponding LCD module are typically provided to a driving IC. Accordingly, the image signal resolution should be converted to that suitable for the LCD module. A conventional LCD source driver will be described with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram of a 192-output, 6-bit gray-scale driving IC of a conventional LCD source driver. As shown, a conventional LCD source driver includes a 64 bit bidirectional shift register 1 for bidirectionally shifting a carry input/output (I/O) signal according to an external clock signal, a 192.times.6 bit latch 2 for successively storing external R, G, and B image signal data (6 bits each) according to the carry I/O signal and outputting stored data according to an external load signal, a 192.times.6 bit digital/analog (D/A) converter 3 for converting an image signal output by latch 2 to an analog signal based on an external POL signal, and data output circuit 4 that output the analog image signal from the D/A converter 3 to a TFT-LCD panel.
FIG. 2 shows in detail the structure of latch 2. As shown, latch 2 comprises two latch sections, a first latch section 2a and a second latch section 2b. Each latch section 2a and 2b is made up of three 192.times.6 bit latches for latching R, G, and B image signals respectively. When first latch section 2a stores image data in response to the external load signal, second latch section 2b outputs its image data into D/A converter 3. Conversely, when second latch section 2b stores image data, first latch section 2a outputs its image data. In this manner, latches 2a and 2b alternately store and output the image data.
In the operation of the conventional LCD source driver, if a VGA LCD module(640.times.480 image) is to be driven, at least 10 driving ICs are needed. This is because a VGA module has 1920 (640.times.3) dots per row (three dots--R (red), G (green), and B (blue), per pixel), but the illustrated driving IC has only 192 data outputs. Similarly, in an LCD module for a XGA (1024.times.768) image, which has 3072 (1024*3) dots per row is to be driven, at least 16 driving ICs are needed (192*16=3072). In summary, the number of needed driving ICs attached to an LCD panel depends on what kind of LCD module is used, and an image signal suitable for the module should be applied to the source driving IC.
If an image signal suitable for the module is applied, latches 2a and 2b store and output data alternately, controlled by the load signal. The data output from latch 2a and 2b is then converted by D/A converter 3 and transferred to the LCD panel by output circuit 4.
The conventional LCD source driver has the following problems. First, since the LCD source driver should be incorporated into a driving IC suitable for the corresponding LCD panel, and image signals suitable for the LCD panel should be used, the multi-scan function cannot be used. Second, when an image signal unsuitable for the module is intended to be displayed without changing or adding driving ICs, an extra converting section is required.